This invention relates to the application of protective coatings to the interior of cans and more particularly, to the application of protective coatings to the interior soldered, welded or adhered overlapped seam or the butt welded seam of a three piece metal can.
Metal cans are made by either one of two processes. One process, the two piece can process, involves drawing a cup from a sheet of metal and subjecting this cup to an ironing press where it is forced through a mandrel forming the cup into a can configuration. The other process, the three piece process involves forming a cylindrical can body from a sheet of metal and then attaching two lids or ends to the opposite ends of the body. The invention of this application is concerned only with the application of protective coatings to three piece cans.
In the manufacture of three piece cans, the cylindrical bodies of the cans are formed by wrapping a sheet of metal around a so-called stubhorn. The ends of the sheet are either butted or overlapped and secured together by either a welded seam, a soldered seam, or a cemented seam as they move longitudinally down the stubhorn. These formed cans travel at an extremely rapid speed down the stubhorn with only a small space between consecutive can bodies.
It is generally the practice in the can industry to apply coating material such as vinyl lacquers onto the inside of the cans to prevent the metallic can bodies from contaminating the contents of the can and to prevent leakage.
Frequently, the entire inside surface of the can is coated. In the alternative, or in addition to this first coating, a different coating can be applied which is limited to the can seam. The present invention is primarily concerned with the application of coating material to the inside seams of three piece cans.
This coating is applied as a spaced line of cans move off or along the stubhorn and past a spray apparatus attached to the end of the stubhorn.
It is preferable for the gun to fire intermittently so that the coating material is sprayed only when a can body is above the nozzle and not at the space between consecutive cans. This prevents excess coating material from fouling up the machinery and also prevents waste.
Prior art guns are typically pneumatically or electrically operated spray guns. The present application is further limited to pneumatically operated spray guns. These prior art pneumatic spray guns are capable of operating with can forming machines producing up to about 400 cans per minute. As an example, at 400 cans per minute, the spray gun must be on for about 140 milliseconds and off for 10 milliseconds and then on again. This 10 milliseconds time for turning off and then on again is the limit of prior art guns. However, can forming machines can operate at much higher speeds. These guns cannot turn off and back on in much less than about 10 milliseconds. Electrically operated guns can function at these speeds, but are undesirable because of heat build up as well as sparking which could ignite the coating material.
Therefore, it is an object of the present invention to coat cans at a rate in excess of 400 cans per minute. Furthermore, it is an object of the present invention to do this using state of the art pneumatic spray guns.
These and other objectives are accomplished by combining two or more spray guns lined up in tandem. The guns are fired alternately so that a single gun does not spray two consecutive cans. The preferred embodiment comprises two guns in tandem which are fired alternately. Thus, a gun turns off as a can is coated and passes by and turns on only after the next can has passed over the gun and the edge of the third can passes over the spray gun. For example, when seven inch can bodies are being coated and are spaced 1/2 inch apart, the on/off time is increased for each gun by 1400%. Thus, by doubling the number of guns, the capacity of the coating apparatus increases 1400%.
Attaching two spray guns to the end of the stubhorn does present the problem of adjusting the nozzle of each gun. It is important particularly with airless spray coating to properly position the spray nozzle with respect to the passing can bodies. Therefore, the present invention also includes means to adjust the position of the nozzle of one gun independently of the second gun.
These and other advantages of this invention will be more readily apparent from the detailed description of the invention.